Carbon dioxide in liquid ammonia: An ab initio Quantum Mechanical/Molecular Mechanical Molecular Dynamics Thermodynamic Integration (QM/MM MD TI) simulation study on structure, dynamics and thermodynamics of solvation

Abstract

In this work, the first investigation of the structural, dynamical and thermodynamical properties of solvated CO2 in liquid ammonia using a recently developed Quantum Mechanical/Molecular Mechanical Molecular Dynamics Thermodynamic Integration (QM/MM MD TI) approach has been carried out. The correlated resolution-of-identity Møller-Plesset Perturbation Theory (RI-MP2) ab initio level of theory has been applied to treat CO2 and its entire first solvation shell. In contrast to aqueous CO2, a negative value for the absolute solvation free energy of −25.9 ± 1.5 kJ/mol was found. This latter value reflects that the solute-solvent interaction is more favorable in liquid ammonia solution. In order to obtain a detailed structural characterisation of solvated CO2, the evaluation of radial distribution functions was performed in conically restricted regions with respect to the main axis of the CO2 molecule. The presented data provides detailed information on the solvation properties of CO2 in pure liquid ammonia. Since knowledge about the latter is a key prerequisite to understand CO2-capture in ammonia solutions, the simulation results can be expected to act as a valuable reference.In addition, the detailed analysis of the individual QM/MM energy contributions showed that the overall potential energy is not affected by artificial contributions, which might influence the determination of solvation free energy.